watchdog: imx2_wdt: Fix set_timeout for big timeout values

The documentated behavior is: if max_hw_heartbeat_ms is implemented, the
minimum of the set_timeout argument and max_hw_heartbeat_ms should be used.
This patch implements this behavior.
Previously only the first 7bits were used and the input argument was
returned.

Signed-off-by: Georg Hofmann <georg@hofmannsweb.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>